Method of manufacturing austenitic stainless steel sheet cast piece

ABSTRACT

Methods for casting an austenitic stainless steel thin strip casting through a continuous caster, e.g., a twin-drum type caster, in which the mold walls move synchronous with the casting to obtain a casting, wherein defects, e.g., salt-and-pepper unevenly glossy defects, on a steel sheet formed after cold rolling or cold forming are prevented. In particular, casting an austenitic stainless steel thin strip casting by regulating a pressing force P of mold wall faces against the casting in the range from more than 1.0 to less than 2.5 t/m, and preferably from more than 1.1 to not more than 1.6 t/m. The continuous caster used may be a twin-drum type continuous caster, with a drum radius R(m) and a pressing force P(t/m) of mold wall faces satisfying the relation 0.5≦(√{square root over (R)})×P≦2.0, and preferably 0.8≦(√{square root over (R)})×P≦1.2.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a national stage application of PCT Application No.PCT/JP03/03891, which was filed on Mar. 27, 2003 and published on Oct.2, 2003 as International Publication No. WO 03/080273 (the“International Application”). This application claims priority from theInternational Application pursuant to 35 U.S.C. § 365. The presentapplication also claims priority under 35 U.S.C. § 119 from JapanesePatent Application No. 2001-335895, filed on Mar. 27, 2003, the entiredisclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to methods for casting an austeniticstainless steel thin strip casting through a continuous caster. Inparticular, casting an austenitic stainless steel thin strip castingthrough a continuous caster, e.g., a twin-drum type caster, in which themold walls move synchronous with the casting to obtain a casting,wherein defects on a steel sheet formed after cold rolling or coldforming are prevented.

BACKGROUND

Synchronous continuous casting processes are processes that do not havea relative speed difference between a casting and the inner walls of amold. For example, such as a twin-drum process (a twin-roll process), atwin-belt process, a single-roll process and the like. A twin-drum typesynchronous continuous casting process is a continuous casting processthat consists of the steps of: (i) pouring molten steel into acontinuous casting mold composed of a pair of cooling drums, which mayhave identical diameters or different diameters and may be disposed inparallel or with an inclination relative to each other, and side weirsfor sealing both end faces of the cooling drums; (ii) forming asolidified shell on the circumference of each of the cooling drums; and(iii) uniting the solidified shells near a position where the rotatingcooling drums come closest to each other (the so-called “kissing point)to form a united thin strip casting.

It is known that surface defects (e.g., unevenly glossy defects on thesurface of a cold-rolled product and rough surface defects on thesurface of a formed product) are sometimes generated along the rollingdirection of a product. For example, surface defects may be formed whenthe product is produced by cold rolling, with hot rolling not appliedbeforehand, and thin strip casting through a twin-drum type continuouscasting process or the like, when cold forming (e.g., draw forming orstretch forming) is applied thereto. These surface defects are generatedin a different manner from the “orange peel phenomenon,” which dependson the diameter of the crystal grains of a cold-rolled product,individually or compositely. In particular, the defects may be in theforms of: (1) small undulated surface defects not more than severalmillimeters in length and not more than 0.5 mm in width on average; and(2) large stream patterned surface defects not more than several hundredmillimeters in length and not more than 3 mm in width on average. Forexample, these surface defects may be observed when a BA product (aproduct produced through bright annealing) is subjected to stretchforming and may deteriorate the appearance of the formed product.

The small undulated surface defects, of not more than severalmillimeters in length and not more than 0.5 mm in width, may begenerated in steel where δ-ferrite remains in an austenite phase. Thesesurface defects may be caused by the uneven structures formed on thesurfaces of a casting as a result of the variation of the residualamount of δ-ferrite due to the heat history of the casting. Thus, thepositions where the surface defects are generated on the top and bottomsurfaces of a steel sheet are not identical with each other. JapanesePatent Publication No. H5-23861, the entire disclosure of which isincorporated herein by reference, discloses a method of preventingsurface defects on a steel sheet product by adjusting the interval ofdimples on the surfaces of the cooling drums. Additionally, JapanesePatent Publication No. H5-293601, the entire disclosure of which isincorporated herein by reference, discloses a method of eliminatingδ-ferrite on the surface layers of a casting by delaying the cooling ofthe casting after it comes out of a high temperature mold. Further,Japanese Patent Publication No. 2000-219919 the entire disclosure ofwhich is incorporated herein by reference, discloses a method comprisingthe steps of: (i) casting a thin strip casting; (ii) imposing a strainto the vicinity of the surfaces of the casting through shot blasting;and (iii) annealing. Thus, recrystallization of the strained surfaceduring annealing creates uniformly sized crystal grains and thereforeremoves the surface gloss.

The large stream patterned surface defects, not more than severalhundred millimeters in length and not more than 3 mm in width, arecaused by the local variation of deformation resistance due to unevendistribution of Ni segregation (e.g., normal segregation and inversesegregation) remaining at the finally solidified portion of a casting,e.g., at a portion in the middle of the thickness of a steel sheetproduct. These surface defects are generated at identical positions onboth the top and bottom surfaces of a steel sheet. Japanese PatentPublication No. H7-268556, the entire disclosure of which isincorporated herein by reference, discloses that Ni segregation ismitigated by performing casting while the degree of superheat ΔT ofmolten steel is controlled to not higher than 50° C. during continuouscasting and thus minimizing the flow of the molten steel at the finallysolidified portion.

Japanese Patent No. 2851252, the entire disclosure of which isincorporated herein by reference, discloses that Ni segregation iscaused by semisolidified molten steel, which is in a state close tofinal solidification and has a solid phase ratio of less than about 1.0,is moved in the direction of the sheet width or in the direction ofcasting by a driving force. This driving force is created by thepressing force P of a mold, imposed when a casting is formed by stickingthe solidified shells together on the mold wall faces. Consequently, Nisegregation may be mitigated and therefore reduce surface defects bydefining the pressing force P as a function of a degree of superheat ΔTof molten steel and controlling the pressing force P to roughly not morethan 5 t/m, and more particularly to controlling the pressing force P toabout 2.5 t/m.

By the various corrective measures described above, the surface defectsgenerated when a product produced by cold-rolling a thin strip castingis subjected to cold forming have been significantly improved. However,it has been found that previously unknown minute surface defects may begenerated. These new surface defects are sometimes recognized asunevenly glossy defects at the stage of a cold-rolled steel sheet in thesame way as before, but are far finer and smaller than the previouslyknown defects. Further, when these new defects are very small they arenot recognized as unevenly glossy defects at the stage of a cold-rolledsteel sheet or after usual cold forming but are found as minute roughsurface defects after excessive cold forming is applied, e.g., deepdrawing or stretch forming, which may cause problems in someapplications. Therefore, these defects must be eliminated in cold-rolledsteel sheet applications, e.g., where buffing after forming is omitted.

As described above, the conventional large stream patterned surfacedefects are generated at identical positions on both the top and bottomsurfaces of a steel sheet. The protrusions and depressions thereof aredistributed in the form of streaks or lines with a height differencebetween a protrusion and a depression of about 1 to 3 μm. A Nisegregation portion is located where the conventional large streampatterned surface defect is generated, with normal segregation andinverse segregation existing in the form of bands in the middle of thesheet thickness. In contrast, although the newly found surface defectsare generated at identical positions on both the top and bottom surfacesof a steel sheet the protrusions and depressions are distributedsporadically and in a zigzag pattern in the form of spots, with a lengthof several tens of millimeters and a height difference betweenprotrusions and depressions of from about 0.1 μm to about 1 μm. Thus,these newly found surface defects have been named “salt-and-pepperunevenly glossy defects” at the stage of a cold-rolled steel sheet. At aportion where a salt-and-pepper unevenly glossy defect is generated inthe middle of the sheet thickness, an Ni inverse segregation portionexists and normal segregation does not exist in the adjacent vicinity.In this respect, a salt-and-pepper unevenly glossy defect isdifferentiated from a conventional rough surface defect where bothnormal segregation and inverse segregation coexist.

SUMMARY OF THE INVENTION

The present invention relates to methods for casting an austeniticstainless steel thin strip casting through a continuous caster. Inparticular, casting an austenitic stainless steel thin strip castingthrough a continuous caster, e.g., a twin-drum type caster, in which themold walls move synchronous with the casting to obtain a casting,wherein defects, e.g., salt-and-pepper unevenly glossy defects, on asteel sheet formed after cold rolling or cold forming are prevented.

According to one embodiment of the present invention, a method forproducing an austenitic stainless steel thin strip casting through acontinuous caster, wherein mold walls move synchronously with thecasting, includes applying a pressing force P of at least one mold wallface against a casting is more than about 1.0 and less than about 2.5t/m. In a further embodiment, the pressing force P of the at least onemold wall face against the casting is more than about 1.1 and less thanabout 1.6 t/m.

According to another embodiment of the present invention, a method forproducing an austenitic stainless steel thin strip casting through acontinuous caster, wherein the mold walls move synchronously with thecasting, the continuous caster is a twin-drum type continuous caster,and the drum radius R(m) and the pressing force P(t/m) of at least onemold wall face satisfies the relation 0.5≦(√{square root over(R)})×P≦2.0. According to a further embodiment of the present invention,the drum radius R(m) and the pressing force P(t/m) of at least one moldwall face satisfies the relation 0.8≦(√{square root over (R)})×P≦1.2.

According to another embodiment of the present invention, the height ofa molten steel pool formed between at least two mold walls is more thanabout 200 mm and less than about 450 mm.

According to another embodiment of the present invention, asolidification time, defined by a span of time between a time when atleast one moving mold wall contacts molten steel to a time when at leasttwo solidified shells unite, is more than about 0.4 second and less thanabout 1.0 second.

According to another embodiment of the present invention, in-linerolling is applied during the process from molding to coiling.

According to another embodiment of the present invention, a degree of Niinverse segregation, defined by the ratio of an amount of Ni at Niinverse segregation portions to an average amount of Ni in an entiresteel is in the range from about 0.90 to about 0.97.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing casting with a twin-drum type continuouscaster;

FIG. 2 is a diagram showing casting with a twin-drum type continuouscaster;

FIG. 3 is a graph showing the relation of the degrees of Ni inversesegregation, the existence of salt-and-pepper unevenly glossy defects,and the pore area ratios to the pressing forces of drums;

FIG. 4 is a graph showing the relation among the drum radiuses R, thepressing forces P, and the existence of salt-and-pepper unevenly glossydefects;

FIG. 5( a) is a perspective sectional view showing the formation ofsalt-and-pepper unevenly glossy defects on a steel sheet after coldrolling and annealing; and

FIG. 5( b) is a perspective sectional view showing the formation ofsalt-and-pepper unevenly glossy defects on a steel sheet aftercold-forming.

DETAILED DESCRIPTION

The mechanism of generating the conventional large stream patternedrough surface defects, not more than several hundred millimeters inlength and not more than 3 mm in width, as discussed above, may becaused by Ni segregation due to the movement of semisolidified moltensteel, i.e., steel in a state close to solidification with a solid phaseratio of approximately less than 1.0, in the direction of the sheetwidth or in the direction of casting, which results from a driving forceand causes rough surface defects, e.g., unevenly glossy defects. Thismechanism can be estimated since Ni normal segregation and Ni inversesegregation coexist adjacently and there is a mass balance of both.

On the other hand, in the case of salt-and-pepper unevenly glossydefects that are the subject of the present invention, as shown in FIG.5, the size of each of the defects are on the order of about severaltens of millimeters in length in the casting direction 20 and severalmillimeters in width. These defects are generated separately from eachother and distributed sporadically, randomly and zigzagged in an area ofabout several hundreds of millimeters in the casting direction andseveral tens of millimeters in the width direction at each portion of acasting 5. The unevenly glossy defects 13 are generated at identicalportions on both the top and bottom surfaces of a casting and a Niinverse segregation portion 12 exists at the portion where an unevenlyglossy defect is generated in a crystal portion 11 that is located atthe middle portion of the sheet thickness. The degree of Ni inversesegregation (the ratio of the amount of Ni at Ni inverse segregationportions to the average amount of Ni in the steel) is roughly not morethan about 0.9. When annealing is applied after cold rolling, as shownFIG. 5( a), a phenomenon is observed wherein the sheet thickness at aportion where an unevenly glossy defect 13 is generated is thinner byabout 0.1 μm as compared to the adjacent portions. This is because theamount of work-induced martensite, formed by cold rolling at an Niinverse segregation portion 12, is larger than at the adjacent portions,and thus volume shrinkage increases at a Ni inverse segregation portion12 after annealing generating a depression. When cold forming, such asstretch forming or draw forming, is applied on top of that, as shown inFIG. 5( b), a phenomenon is observed wherein the sheet thickness at aportion where an unevenly glossy defect 13 is generated is thicker byabout 1 μm as compared to the adjacent portions. This is because plasticdeformation is uneven during forming due to the unevenness of themartensite amount as stated above. As a result, a salt-and-pepperunevenly glossy defect is generated at a portion corresponding to a Niinverse segregation portion on the surface of a steel sheet afterforming.

Since uneven plastic deformation during forming functions ratherstrongly, as compared to volume shrinkage after annealing in theaforementioned mechanism, the height difference between a protrusion anda depression will be larger for uneven plastic deformation. Therefore,depending on the degree of Ni inverse segregation, there may be asituation where Ni inverse segregation becomes significant after coldforming. For example, rough surface defects may appear after coldforming even though such defects are not present in a steel sheet aftercold rolling and annealing. In such a situation conventional largestream patterned surface defects, not more than several hundredmillimeters in length and not more than 3 mm in width, may be a problem.

Ni segregation (normal segregation and inverse segregation) that causessurface defects may be improved by evaluating the amount of Ni, forexample, roughly in a region of 25 μm in the thickness direction and 500μm in the width direction at a segregation portion in the case ofconventional large stream patterned surface defects, for example, asdisclosed in Japanese patent No. 2851252, the entire disclosure of whichis incorporated herein by reference. However, since salt-and-pepperunevenly glossy defects appear very minutely and sporadically, it may bedifficult to evaluate segregation by this method since it may benecessary to evaluate Ni amount in detail over a wider range. Inparticular, it may be necessary to evaluate Ni segregation in a range ofabout several millimeters in the width direction in the case ofsalt-and-pepper unevenly glossy defects.

On the basis of the aforementioned nature of salt-and-pepper unevenlyglossy defects, the mechanism of generating a Ni inverse segregationportion at the middle portion of the sheet thickness can be estimated asfollows. When molten steel begins to solidify by contacting with moldwalls immediately under a meniscus, as molten steel components includingNi in a liquid phase do not yet begin to concentrate, the concentrationof each component in an initial solidification structure is basically inthe state of inverse segregation, depending on the distributioncoefficient of each component. The initial solidification structure iscooled directly by the mold walls, thus the speed of solidification ishigh and therefore a structure composed of chilled crystals is formed.When solidification proceeds, the components on the liquid phase side ofthe interface between the solid phase and the liquid phase concentrate,whereas the concentration of the components on the solid phase side areequal to the initial concentrations of the components in molten steel.

In addition, during solidification the structure transforms from chilledcrystals to columnar crystals. It is known that such chilled crystals ofNi inverse segregation generated immediately under a meniscus, asdescribed above, tend to separate from solidified shells and turn tofree chilled crystals, based on a function of compositional supercoolingat the interface between a solid phase and a liquid phase. The freechilled crystals are suspended in a supercooling zone, or massy zone, onthe liquid phase side of the interface between a solid phase and aliquid phase, and move together with solidified shells formed along themold walls, and reach a kissing point where both the left and rightsolidified shells contact with each other and are united. An equiaxedcrystal region (i.e., a solid and liquid coexisting region) is formedwith chilled crystals acting as nuclei right above the kissing point.

When a material balance is reached between the upper part and the lowerpart of a kissing point, free chilled crystals of Ni inverse segregationthat have reached the middle portion of a sheet thickness right above akissing point are fed, together with equiaxed crystals, to the middleportion of the sheet thickness while accompanying solidified shells and,as a result, inverse segregation regions are formed at the middleportion of the sheet thickness uniformly in the directions of the widthand length.

On the other hand, when a material balance is disturbed between theupper part and the lower part of a kissing point and equiaxed crystalregions, wherein a solid phase and a liquid phase coexist, are not fedto the middle portion of a sheet thickness, substances containingchilled crystals of Ni inverse segregation accumulate right above thekissing point. When such accumulated substances are trapped insolidified shells irregular Ni inverse segregation regions may be formedat the portion where the accumulated substances are trapped in themiddle portion of a sheet thickness and the trapped portions may bedifferentiated from the other portions. Since the substances trapped inthe solidified shells occurs randomly in the directions of the width andlength of a casting, the Ni segregation portions at the middle portionof a sheet thickness may cause salt-and-pepper unevenly glossy defects.

According to the present invention the material balance between theupper part and the lower part of a kissing point may depend on thepressing force of the mold wall faces at the kissing point and, in theregion of the pressing force substances containing chilled crystals ofNi inverse segregation tend to accumulate right above the kissing point.Therefore, by using the appropriate pressing force the accumulation ofthe substances containing chilled crystals of Ni inverse segregation maybe reduced or eliminated. As a result, Ni inverse segregation portionsthat exist in the salt-and-pepper state at the middle portion of a sheetthickness are removed and the generation of salt-and-pepper unevenlyglossy defects is eliminated.

Salt-and-pepper unevenly glossy defects may appear with a mold wall facepressing force P of 2.5 t/m. Thus, according to one embodiment of thepresent invention, generation of salt-and-pepper unevenly glossy defectsmay be reduced by controlling a pressing force P to less than about 2.5t/m. According to another embodiment of the present invention,generation of salt-and-pepper unevenly glossy defects may be reduced bycontrolling a pressing force P to less than about 1.6 t/m. As usedherein, the pressing force P(t/m) is a value obtained by dividing awhole pressing force (t) of a mold wall face by the mold width (m), andthus is defined as the pressing force per unit mold width. For example,a mold width equals a drum width in the case of a twin-drum typecontinuous caster.

However, when the pressing force is excessively small, center poresappear at the middle portion of the sheet thickness of a casting. Inparticular, center pores may appear when a pressing force P of 1.0 t/mis used. Thus, according to one embodiment of the present inventioncenter pores may be reduced or eliminated by using a pressing force P ofmore than about 1.0 t/m. According to another embodiment of the presentinvention, center pores may be further reduced or eliminated by using apressing force P of more than about 1.1 t/m. In yet another embodimentof the present invention, center pores may be further reduced oreliminated by using a pressing force P of more than about 1.2 t/m.

In the case where a continuous caster is a twin-drum type continuouscaster, a preferable result can be obtained by specifying a pressingforce P of mold wall faces in accordance with a drum radius R. Inparticular, a good result may be obtained according to the presentinvention by regulating a drum radius R(m) and a pressing force P(t/m)of the mold wall faces in terms of the range of the value (√{square rootover (R)})×P.

As discussed above, when the pressing force is too large, Ni inversesegregation appears at the middle portion of a sheet thickness. As adrum radius increases, the region of molten pool adjacent to a kissingpoint deepens with the upper part thereof narrowing and equiaxedcrystals tend to accumulate with chilled crystals of Ni inversesegregation acting as nuclei. Therefore, as the drum radius increasesthe upper limit in the appropriate range of a pressing force beyondwhich salt-and-pepper unevenly glossy defects appear shifts toward alower value. In contrast, as the drum radius decreases, the region ofmolten pool adjacent to a kissing point becomes shallower with the upperpart thereof widening and equiaxed crystals hardly accumulate withchilled crystals of Ni inverse segregation acting as nuclei. Therefore,as the drum radius decreases the upper limit in the appropriate range ofa pressing force beyond which salt-and-pepper unevenly glossy defectsappear shifts toward a higher value.

On the other hand, when the pressing force is too small, there arises aproblem of abnormal casting including the generation of center pores. Inparticular, as the drum radius decreases, the molten steel pool betweendrums shallows and thus the fluctuation of a molten steel surfaceincreases. Therefore, the variation of solidified shell thicknessincreases over the direction of the sheet width. Furthermore, as thevariations of reactive forces in the direction of drum width increases,for the above reasons, the casting operation shifts toward an unstableoperation. Therefore, as the drum radius decreases the lower limit of apressing force beyond which an abnormal casting occurs shifts toward ahigher value. In contrast, as the drum radius increases, the variationof reactive force in the direction of drum width decreases and thestability of casting operation improves. Therefore, as the drum radiusincreases the lower limit of a pressing force beyond which an abnormalcasting occurs shifts toward a lower value.

The present inventors intensively carried out studies by properlychanging a drum radius R(m) and a pressing force P(t/m). According tothe present invention the appropriate regions of a drum radius and apressing force beyond which salt-and-pepper unevenly glossy defectsoccurs may be specified by the term √{square root over (R)}×P. In oneembodiment of the present invention, a method of casting may includeregulating a drum radius R(m) and a pressing force P(t/m) to satisfy therelation 0.5≦(√{square root over (R)})×P≦2.0. In another embodiment ofthe present invention, a method of casting may include regulating a drumradius R(m) and a pressing force P(t/m) to satisfy the relation0.8≦(√{square root over (R)})×P≦1.2.

In the case of a twin-drum type continuous caster for instance, as shownin FIG. 2, a molten steel pool 2 is formed on the space surrounded by apair of drums 1 and side weirs to seal both end faces of the drums.There exists a range of heights H of the molten steel pool 2 in whichsalt-and-pepper unevenly glossy defects are minimized. As definedherein, the height H of a molten steel pool 2 is the distance from akissing point 4 to a molten steel surface 7 as shown in FIG. 2. When thepool height H is less than 200 mm, though the time during which chilledcrystals generated at a meniscus 8 grow is short, most of the grownchilled crystals accumulate directly to a kissing point 4 and thereforesalt-and-pepper unevenly glossy defects are apt to be generated. Inaddition, when the pool height H is greater than 450 mm, though most ofthe chilled crystals generated at a meniscus 8 disperse and remelt in amolten steel pool, some surviving chilled crystals become large sincethey have enough time to grow, and the amount thereof accumulated at akissing point 4 increases. Therefore, salt-and-pepper unevenly glossydefects are apt to be generated. Accordingly, a good result can beobtained by regulating a molten steel pool height H in the range fromnot less than about 200 mm to not more than about 450 mm.

The solidification time t is defined as the time between when the movingmold walls contact with molten steel at a meniscus 8 to the time whensolidified shells 3 of both sides unite at a kissing point 4. Thesolidification time t may be determined by the shape of a molten steelpool 2 and the traveling speed of the mold walls. There exists in asolidification time t a range appropriate for producing a castingwherein salt-and-pepper unevenly glossy defects are minimized. When thesolidification time t is shorter than 0.4 second, though the time duringwhich chilled crystals generated at a meniscus grow is short, most ofthe grown chilled crystals accumulate directly to a kissing point 4 andtherefore salt-and-pepper unevenly glossy defects are apt to begenerated. In addition, when the solidification time t exceeds 1.0second, though most of the chilled crystals generated at a meniscus 8disperse and remelt in a molten steel pool, some surviving chilledcrystals become large since they have a time enough to grow, the amountthereof accumulated to a kissing point 4 increases, and thereforesalt-and-pepper unevenly glossy defects are apt to be generated.Accordingly, a good result can be obtained by regulating asolidification time t, the time from when the moving mold walls contactwith molten steel to the time when the solidified shells of both sidesunite, in the range from not shorter than about 0.4 second to not longerthan about 1.0 second.

As explained above, as a pressing force P of the mold wall facesdecreases, which suppresses the generation of salt-and-pepper unevenlyglossy defects, abnormal casting including the generation of centerpores is apt to occur. According to the present invention, it ispossible to carry out casting stably with a small pressing force byapplying in-line rolling during the process from molding to coiling, andthus bonding center pores with pressure. Though the situation variesdepending on the composition of steel to be cast or the type of casterand drums, as long as rolling is applied to a casting with enoughpressure and at a sufficiently high temperature to bond the centerpores, it is possible to eliminate the effect of the center pores. Inparticular, as shown in FIG. 1, it is preferable to install an in-linerolling mill 6 at a place downstream of the drums 1, in which thetemperature of a casting is not lower than about 1,000° C. and applyrolling under the condition of reducing a thickness by not less thanabout 10% in terms of a sheet thickness ratio. Thus, as long as thecenter pores can bond, the pressure and rolling conditions are notrestricted except by the temperature at which rolling is applied. Centerpores tend to appear when a pressing force is weak and therefore thecenter pores may be by applying in-line rolling. Therefore, according toone embodiment of the present invention, a casting may be cast whereinthe center pores are minimized by regulating a pressing force to morethan 1.0 t/m. In particular, according to another embodiment of thepresent invention, it may be preferable to regulate a pressing force tomore than 1.1 t/m to minimize the generation of center pores. In stillanother embodiment of the present invention, it may be preferable toregulate a pressing force to more than 1.2 t/m.

EXAMPLE

A twin-drum type continuous caster as shown in FIG. 1 may be usedaccording to the present invention. The width of each of the drums 1 was1,000 mm, the thickness of each of the castings 3 mm, and the steelgrade of each of the castings AISI 304 steel (austenitic stainlesssteel). The radius R of each of the drums 1 was 0.6 m in each exampledescribed below, except Example 2. The pool height H was 350 mm in eachexample described below, except Example 3. The solidification time t was0.7 second in each example described below, except Example 4. When adrum radius R, a pool height H and a solidification time t are changedfrom the above values, the respective values are expressed in therelevant tables of the following examples.

In-line rolling was not applied in Examples 1 to 4 below, but the casesof applying and not applying in-line rolling were compared in Example 5below. When in-line rolling was applied, the in-line rolling mill 6shown in FIG. 1 was used for the rolling. The temperature of a castingat the entry of the rolling mill was 1,220° C. when in-line rolling wascarried out. A reduction ratio of the in-line rolling was defined by theexpression (the thickness of a casting—the thickness thereof afterin-line rolling)/the thickness of a casting×100 in terms of percentage.

The castings that were cast were cold-rolled to the thickness of 1.0 mmand thereafter subjected to stretch forming to form the shape of acylinder 50 mm in diameter as cold forming. In that case, two kinds ofstretch forming was applied; light forming of 5 mm in stretch height andheavy forming of 30 mm in stretch height.

The degree of Ni inverse segregation was obtained by measuring an Niamount over a region 100 μm in thickness direction and 1 cm in widthdirection at the middle portion of the thickness on the cross section inthe direction of the width of a casting with an X-ray microanalyzer andcalculating the ratio of Ni amount in the region to the Ni amount in aladle (i.e., the amount of Ni in molten steel).

Salt-and-pepper unevenly glossy defects were determined by visuallyobserving the surfaces of the specimens at the stage of cold-rolledsteel sheets and after cold forming (both light forming and heavyforming). When salt-and-pepper unevenly glossy defects were conspicuousno further examination was necessary. When salt-and-pepper unevenlyglossy defects were questionable, minute protrusions and depressionswere determined as the unevenness of polish by scrubbing the surfacewith abrasive paper of about #1,000 in mesh. In any of the cases,spot-shaped or spindle-shaped patterns that were distributed in a zigzagwere judged as salt-and-pepper unevenly glossy defects.

The area ratio of center pores was obtained by calculating the ratio (%)of the total area of center pores in the area of one square meter on thesurface of a casting on the basis of radioparency photography.

Example 1

As shown in Table 1, the pressing forces P of the drums were varied inthe range from 1.0 to 2.6 t/m, and the degrees of Ni inversesegregation, the existence of salt-and-pepper unevenly glossy defectsand the center pore area ratios of the steel sheets were evaluated. Theresults are shown also in FIG. 3. In the case of No. 2 according to thepresent invention, the pressing force P was 1.1 t/m, no salt-and-pepperunevenly glossy defects appeared, which is good and, though center poreswere generated at 2.5% in terms of an area ratio, the value was a levelapplicable to practical use. In the cases of Nos. 7 and 8 according tothe present invention, the pressing forces P were 1.8 to 2.4 t/m and,though salt-and-pepper unevenly glossy defects appeared after subjectedto heavy forming in cold forming, no salt-and-pepper unevenly glossydefects appeared at the stage of cold-rolled steel sheets and afterlight forming in cold forming. In the cases of Nos. 3 to 6 according tothe present invention, the pressing forces P were in the range from 1.2to 1.6 t/m, no salt-and-pepper unevenly glossy defects appeared, thecenter pore area ratios were 0%.

In case of No. 1 that was a comparative example, the pressing force Pwas 1.0 t/m and center pores were generated by 6.3% in terms of an arearatio. In the cases of Nos. 9 and 10 which were comparative examples,the pressing forces P were from 2.5 to 2.6 t/m and salt-and-pepperunevenly glossy defects appeared at the stage of cold-rolled steelsheets and also after cold forming.

Example 2

As shown in Table 2, the drum radiuses R were varied in the range from0.2 to 0.8 m and the pressing forces P were varied at 4 levels, and thenthe existence of salt-and-pepper unevenly glossy defects and therelation between the center pore area ratios and the values (√{squareroot over (R)})×P of the steel sheets were evaluated. The results areshown also in FIG. 4. The curves drawn in FIG. 4 are the ones that haverespective identical (√{square root over (R)})×P values; from above,(√{square root over (R)})×P=2.2 (the upper broken line), (√{square rootover (R)})×P=1.2 (the upper solid line), (√{square root over (R)})×P=0.8(the lower solid line) and (√{square root over (R)})×P=0.5 (the lowerbroken line).

In the cases of Nos. 12 to 21 according to the present invention, thevalues (√{square root over (R)})×P were in the range from 0.8 to 2.0 anda good result was obtained in any of the cases. In the case of No. 11according to the present invention, the value (√{square root over(R)})×P was 0.5 and, though the center pore area ratio was 1.4%, thevalue was a level applicable to practical use. In the case of No. 22that was a comparative example, the value (√{square root over (R)})×Pwas 2.3 and the salt-and-pepper unevenly glossy defects were observed atthe stage of the cold-rolled steel sheet and also after cold forming.

Example 3

As shown in Table 3, the molten steel heights H were varied in the rangefrom 190 to 460 mm, the pressing forces P of the drums were fixed to 1.5t/m, and then the existence of salt-and-pepper unevenly glossy defectsof the steel sheets was evaluated. In the cases of Nos. 24 to 26, themolten steel heights H were in the appropriate range from 200 to 450 mmand salt-and-pepper unevenly glossy defects were not observed. In thecases of Nos. 23 and 27, as the molten steel heights H were outside theappropriate range, the salt-and-pepper unevenly glossy defects wereobserved.

Example 4

As shown in Table 4, the solidification times t were varied in the rangefrom 0.3 to 1.1 seconds, the pressing forces P of the drums were fixedto 1.5 t/m, and then the existence of salt-and-pepper unevenly glossydefects of the steel sheets was evaluated. In the cases of Nos. 29 to33, the solidification times t were in the appropriate range from 0.4 to1.0 second and salt-and-pepper unevenly glossy defects were notobserved. In the cases of Nos. 28 and 34, as the solidification times twere outside the appropriate range, the salt-and-pepper unevenly glossydefects were observed.

Example 5

As shown in Table 5, the pressing forces P of the drums were fixed to1.1 t/m, in-line rolling was applied with the reduction ratios thereofvaried or was not applied, and then the existence of salt-and-pepperunevenly glossy defects and the center pore area ratios of the steelsheets were evaluated. In the case of No. 35, as in-line rolling was notapplied, the center pore area ratio was 2.5%. In the case of No. 36,in-line rolling was applied at the reduction ratio of 8% and the centerpore area ratio was 8%. In the case of No. 37, the in-line rolling wasapplied at the reduction ratio of 10% and the center pore area ratio was0%, resulting in a good result. Salt-and-pepper unevenly glossy defectsdid not appear in any of the above cases and good results could beobtained.

TABLE 1 Salt-and-pepper unevenly glossy defect Center Pressing Degree ofNi Cold forming pore force P; inverse Cold-rolled Light Heavy area No.t/m segregation steel sheet forming forming ratio; % Remarks 1 1.00.95–0.97 Nil Nil Nil 6.3 Comparative example 2 1.1 0.95–0.97 Nil NilNil 2.5 Invented example 3 1.2 0.95–0.97 Nil Nil Nil 0 Invented example4 1.3 0.94–0.96 Nil Nil Nil 0 Invented example 5 1.5 0.93–0.96 Nil NilNil 0 Invented example 6 1.6 0.92–0.95 Nil Nil Nil 0 Invented example 71.8 0.92–0.94 Nil Nil Present 0 Invented example 8 2.4 0.90–0.93 Nil NilPresent 0 Invented example 9 2.5 0.88–0.91 Present Present Present 0Comparative example 10 2.6 0.87–0.90 Present Present Present 0Comparative example

TABLE 2 Salt-and-pepper unevenly glossy defect Pressing Drum Coldforming Center force P; radius Cold-rolled Light Heavy pore area No. t/mR; m {square root over (R · P)} steel sheet forming forming ratio; %Remarks 11 1.1 0.2 0.5 Nil Nil Nil 1.4 Invented example 12 1.8 0.2 0.8Nil Nil Nil 0 Invented example 13 2.6 0.2 1.2 Nil Nil Present 0 Inventedexample 14 1.5 0.4 0.9 Nil Nil Nil 0 Invented example 15 1.8 0.4 1.1 NilNil Nil 0 Invented example 16 2.6 0.4 1.6 Nil Nil Present 0 Inventedexample 17 1.5 0.6 1.2 Nil Nil Nil 0 Invented example 18 1.8 0.6 1.4 NilNil Present 0 Invented example 19 2.6 0.6 2.0 Nil Nil Present 0 Inventedexample 20 1.5 0.8 1.3 Nil Nil Present 0 Invented example 21 1.8 0.8 1.6Nil Nil Present 0 Invented example 22 2.6 0.8 2.3 Present PresentPresent 0 Comparative example

TABLE 3 Salt-and-pepper Molten unevenly glossy defect Drum steel Coldforming Pressing radius height Cold-rolled Light Heavy No. force P; t/mR; m H; mm steel sheet forming forming 23 1.5 0.6 190 Nil Nil Present 241.5 0.6 210 Nil Nil Nil 25 1.5 0.6 350 Nil Nil Nil 26 1.5 0.6 440 NilNil Nil 27 1.5 0.6 460 Present Present Present

TABLE 4 Salt-and-pepper unevenly glossy defect Drum Solidification Coldforming Pressing radius R; time Cold-rolled Light Heavy No. force P; t/mm t; second steel sheet forming forming 28 1.5 0.6 0.3 Nil Nil Present29 1.5 0.6 0.4 Nil Nil Nil 30 1.5 0.6 0.5 Nil Nil Nil 31 1.5 0.6 0.7 NilNil Nil 32 1.5 0.6 0.9 Nil Nil Nil 33 1.5 0.6 1.0 Nil Nil Nil 34 1.5 0.61.1 Present Present Present

TABLE 5 Salt-and-pepper unevenly glossy defect Drum In-line Cold formingCenter Pressing radius reduction Cold-rolled Light Heavy pore area No.force P; t/m R; m ratio; % steel sheet forming forming ratio; % 35 1.10.6 0 Nil Nil Nil 2.5 36 1.1 0.6 8 Nil Nil Nil 1.1 37 1.1 0.6 10 Nil NilNil 0

The present invention, in a method of casting an austenitic stainlesssteel thin strip casting with a continuous caster wherein mold wallsmove synchronously with the casting, makes it possible to preventsalt-and-pepper unevenly glossy defects distributed zigzag in the formof spots from appearing on a steel sheet after cold rolling and coldforming by regulating a pressing force P of mold wall faces in theappropriate range from more than about 1.0 to less than about 2.5 t/m.

1. A method for producing an austenitic stainless steel thin stripcasting through a continuous caster wherein mold walls movesynchronously with the casting, comprising applying a pressing force Pof the at least one mold wall face against the casting wherein thepressing force is more than about 1.1 and less than about 1.6 t/m. 2.The method of claim 1 wherein a height of a molten steel pool formedbetween at least two mold walls is more than about 200 mm and less thanabout 450 mm.
 3. The method of claim 1 wherein a solidification time,defined by a span of time between a time when at least one moving moldwall contacts molten steel to a time when at least two solidified shellsunite, is more than about 0.4 second and less than about 1.0 second. 4.The method of claim 1 wherein in-line rolling is applied during theprocess from molding to coiling.
 5. The method of claim 1 wherein adegree of Ni inverse segregation, defined by the ratio of an amount ofNi at Ni inverse segregation portions to an average amount of Ni in anentire steel is in the range from about 0.90 to about 0.97.
 6. A methodfor producing an austenitic stainless steel thin strip casting through acontinuous caster wherein mold walls move synchronously with the castingwherein the continuous caster is a twin-drum type continuous caster, andwherein the drum radius R (m) and the pressing force P (t/m) of at leastone mold wall face satisfies the relation 0.8≦(√{square root over(R)})×P≦2.0.
 7. The method of claim 6 wherein a height of a molten steelpool formed between at least two mold walls is more than about 200 mmand less than about 450 mm.
 8. The method of claim 6 wherein asolidification time, defined by a span of time between a time when atleast one moving mold wall contacts molten steel to a time when at leasttwo solidified shells unite, is more than about 0.4 second and less thanabout 1.0 second.
 9. The method of claim 6 wherein in-line rolling isapplied during the process from molding to coiling.
 10. The method ofclaim 6 wherein a degree of Ni inverse segregation, defined by the ratioof an amount of Ni at Ni inverse segregation portions to an averageamount of Ni in an entire steel is in the range from about 0.90 to about0.97.
 11. A method for producing an austenitic stainless steel thinstrip casting through a continuous caster wherein mold walls movesynchronously with the casting, comprising applying a pressing force Pof the at least one mold wall face against the casting is more thanabout 1.1 and less than about 1.6 t/m, and the drum radius R (m) and thepressing force P (t/m) of at least one mold wall face satisfies therelation 0.8≦(√{square root over (R)})×P≦2.0.